The present invention relates to methods of testing logic systems, and logic systems in which the methods are applied.
The system according to the invention is advantageous in that it allows a logic state or states to be modified or detected in a signal-processing system whose interconnecting circuits are not directly accessible from the exterior.
The invention is applicable in particular to the checking and use of large-scale integrated circuits, normally referred to as LSI circuits, and to equipment which makes use of LSI circuits.
So-called large-scale integrated circuits owe their name to the fact that they comprise a very large number of individual circuits which are deposited on one chip by integrated circuit techniques.
The use of such circuits has grown very rapidly in all fields of elecronics. Among their many important advantages are the following:
(1) High performance; PA1 (2) Good reliability; PA1 (3) Small size; and PA1 (4) Moderate manufacturing cost.
However, these advantages are counterbalanced by disadvantages which make it necessary for manufacturers to take precautions both in using them and in designing them.
It is therefore necessary to find methods and means for allowing effective tests to be performed on LSI integrated circuits and on equipment using them.
These tests need to recognize all the faults which are present even though, as one moves further away from individual components, it becomes increasingly difficult, if not impossible, to detect and locate faults.
It will be apparent from the above that the methods and means of testing are of prime importance in making sensible use of LSI circuits.
There is also another aspect of modern data processing circuits which needs to be mentioned with regard to testing operations.
The miniaturization made possible by modern technology has enabled manufacturers to produce more complex circuits without any excessive increase in the size and cost of the equipment. Taking advantage of this, one of the first applications which was thought of is the production of fault-proof systems. Accordingly, there are self-repairing systems of various kinds in existence.
One known type of equipment makes use of circuit redundancy. In the event of one or more circuits failing, additional circuits take the place of the faulty circuits. One example is so-called "majority logic" equipment.
Another type of equipment makes use of redundancy in the signals processed. The signals include additional states which enable checking and repair circuits to correct the consequences of faulty operation by certain circuits.
With both of these equipment types, a number of circuits may be faulty without any external sign of faulty operation being apparent. The equipment conceals its faults.
In operations involving the testing of equipment which includes known self-correcting arrangements to detect faults, it is essential to have means capable of detecting all the faults, including those which have been dealt with by self-correction.
An already stated, LSI circuits are in fact carried on a chip of small dimensions. Given this fact, in order not to lose the benefit of miniaturization and in order not to detract from reliability, LSI circuits have only a small number of external connections. At the present time the number of connections, also termed terminals, is of the order of fifty.
If it is considered that one chip may contain several hundred circuits, it is clear that from the terminals it is generally possible only to reach the circuits positioned at the two electrical extremities of the chip, that is to say at its input and output. Generally, under no circumstances is it possible to gain access to the circuits or interconnections situated at the heart of the chip, given their extreme density.
This disadvantage also extends to modules which employ groups of chips since these modules, for the same reasons as above, have only a limited number of access terminals. In summary, it can be said that if no special provisions are made, it is not possible to gain access from the exterior to intermediate circuits situated at the heart of chips or modules. This is a serious disadvantage when equipment of this type is to be tested to detect any breakdowns or faults which it may have.
The need for testing arises both during the manufacture of LSI circuits and during their subsequent use. To ensure the quality of his products, a manufacturer has to check the satisfactory operation of all the circuits. This is the price he has to pay to be sure of obtaining a faultless product.
Various publications have been devoted to testing, diagnosis and maintenance techniques. Reference may be made to the following books: "Fault Diagnosis of Digital Systems" authored by Chang, Manning and Metze, and published by Wiley in 1970; and "Fault Detection in Digital Circuits" authored by Friedman and Menon, and published by Prentice Hall in 1971.
More recent developments in techniques for detecting and locating faults are reviewed in the journal "Computer" for June 1976 in an article authored by Bennets and Scott entitled "Recent Developments in the Theory and Practice of Testable Logic Design."
As has just been seen, in the case of complex circuits having interconnecting networks which are inaccessible from the exterior, as they are in the case of self-repairing circuits, prior art techniques do not allow all the faults which may occur in such circuits to be detected and located.